While a variety of dewatering aids are employed for concentration of sludges in industrial waste water and household waste water, a method is known as one of them which is the use of waste paper, for example, in which low-grade waste paper and colored waste paper of high utilization value for reclaimed paper are crushed to have a particle diameter of about 0.25 to 1 mm to be admixed in a proportion of 10 to 50% by mass relative to the solid matter in the sludge and dewatering is conducted under a squeezing pressure of 0.05 to 0.30 MPa by utilizing a belt press or a screw press (see non-patent documents 1 and 2).
Besides, proposals were made heretofore for the dewatering method in which sludge is admixed with a coagulant and crushed waste paper to form a coagulated sludge and this coagulated sludge is supplied to a belt press-type dewatering machine provided with a cloth in circulatory running between a gravity dewatering part and a compression dewatering part to effect dewatering wherein the blending amount of waste paper and the running speed of the cloth are controlled to stabilize the water content of the dewatered cake (see patent document 1), a sludge treatment method in which an organic sludge is blended with 1 to 20 weight % of disintegrated pulp relative to the sludge SS followed by a concentration procedure (see patent document 2), a dewatering method of sludges in which the sludge is coagulated by admixing-with a coagulant and the coagulated sludge is subjected to primary dewatering followed by blending of waste paper as a dewatering aid to be subjected to secondary dewatering (see patent document 3), the composting method in which the sludge produced in an excrement processing yard is admixed with an inorganic salt as a coagulant to undertake dewatering followed by the admixture of the dewatered cake with pulverized shells to adjust the pH and further addition of crushed waste paper to improve the air permeability for multiplication of aerobic microorganisms so as to effect decomposition of the organic matter and absorption of the malodor (see patent document 4) and others.
It is almost impossible in these sludge-dewatering methods by utilizing waste paper, however, that the water content of sludgy animal feces or sewage sludges and sludges from industrial waste water should be decreased from 90% by mass or higher to 70% by mass or lower and, if possible, the sludge must be blended with at least 20% by mass or, depending on the case, at least 30% by mass of waste paper relative to the sludge so that handling thereof is unavoidably troublesome due to an increase of the amount of the dewatered cakes as the defective points.    Patent document 1: Japanese Patent Kokai 9-216000 (claims and elsewhere)    Patent document 2: Japanese Patent Kokai 11-19698 (claims and elsewhere)    Patent document 3: Japanese Patent Kokai 2001-121199 (claims and elsewhere)    Patent document 4: Japanese Patent Kokai 2001-302377 (claims and elsewhere)    Non-patent document 1: “Journal of Sewerage, Monthly” vol. 21, No. 73, pp 80-86    Non-patent document 2: “Journal of Resources and Environment” 1988, vol. 34, No. 9, pp 25-33